It is a non-invasive medical imaging technique to map the curvature of the corneal surface and the eye’s external structure.
Since the cornea usually is responsible for approximately 70% of the refractive power of the eye, its topography is of vital importance in determining the quality of vision and corneal health.
The three-dimensional map is, therefore, a valuable aid to the examining ophthalmologist or optometrist and can aid in the diagnosis and treatment of several conditions; in cataract surgery planning and intraocular lens implantation in refractive surgery planning.
In the development of keratoscopy, corneal topography extends the measurement range of the four points separated by a few millimeters that keratometry offers to a grid of thousands of points covering the entire cornea. The procedure takes seconds to complete and is entirely painless.
Corneal topography is also known as photokeratoscopy or videokeratography.
History of corneal topography
The corneal topographer owes his heritage to the Portuguese ophthalmologist Antonio Placido, who, in 1880, saw a painted disk (Placido disk) of alternating black and white rings reflected in the cornea.
The rings showed as contour lines projected on the corneal tear film. Javal L., a pioneer in the field in the 1880s, incorporated the rings into his ophthalmometer and mounted an eyepiece that magnified the eye’s image.
He proposed that the image be photographed or represented in diagram form to allow an analysis of the picture.
In 1896, Allvar Gullstrand incorporated the disk into his ophthalmoscope, examining photographs of the cornea through a microscope and calculating the curvature using a numerical algorithm manually.
Gullstrand recognized the technique’s potential and commented that, despite its laboriousness, it could “give a resulting precision that previously could not be obtained in any other way.”
The flat field of the Placido disk reduced accuracy near the corneal periphery, and in the 1950s, the Wesley-Jessen company made use of a curved bowl to reduce field defects.
The cornea’s curvature could be determined by comparing photographs of the rings with standardized images.
In the 1980s, photographs of projected images were digitized by hand and then analyzed by a computer. Automation of the process soon followed with the image captured by a digital camera and passed directly to a computer.
The patient is seated in front of a bowl that contains an illuminated pattern, usually a series of concentric rings. The design is focused on the anterior surface of the patient’s cornea and reflects a digital camera in the center of the bowl.
The topology of the cornea is revealed by the shape taken by the reflected pattern. A computer provides the necessary analysis, generally determining the position and height of several thousand points across the cornea.
The topographic map can be rendered in various graphic formats, such as a sagittal map, which color-codes the steepness of the curvature-based on its diopter value.
Uses of corneal topography
Computerized corneal topography can be used for diagnosis. It is, in fact, one of the examinations that patients must undergo before crosslinking and mini asymmetric radial keratotomy.
For example, the KISA% index (keratometry, IS, oblique percentage, astigmatism) is used to arrive at a diagnosis of keratoconus to detect suspected keratoconus patients and to analyze the degree of changes in corneal inclination in healthy relatives.
However, the topography itself is a measure of the first reflective surface of the eye (tear film). It does not provide additional information next to the shape of this layer expressed in curvature.
Keratoconus is a pattern of the entire cornea. Therefore each measurement simply focusing on one layer may not be sufficient for a state-of-the-art diagnosis.
Especially early cases of keratoconus can be missed by simple topographic measurement, which is critical when considering refractive surgery.
The measurement is also sensitive to unstable tear films. Also, measurement alignment can be difficult, especially with eyes with keratoconus, significant astigmatism, or sometimes after refractive surgery.
Corneal topography instruments generate a simulated keratometry measurement (SimK), which approximates the classic size of the widely used keratometer.
Another novel use of corneal topographic data is called CorT, which has been shown to quantify refractive astigmatism more accurately than SimK and other approaches. Cora uses data from all Placido rings across the cornea compared to SimK, based on a single call.
While corneal topography relies on light reflected from the front (anterior) part of the cornea, a technique called corneal tomography also measures the posterior shape of the cornea.